Overview

The NE555 is the single most iconic hobbyist IC: a precision timer that makes one-shot delays (monostable) or free-running oscillators / PWM (astable) with a handful of resistors and a capacitor. Internally it compares the capacitor voltage against thresholds at roughly ⅓ VCC (trigger) and ⅔ VCC (threshold), then drives a flip-flop that controls the output and the discharge transistor.

Output can sink or source up to 200 mA, so it can drive LEDs, small buzzers, or logic inputs directly. Commercial clones and second sources (LM555, SA555, CMOS TLC555 / LMC555) share the same pinout with different supply/current/temp trade-offs.

Quick reference

FunctionPrecision timer (monostable / astable)
Supply range4.5 V – 16 V (NE555; SE555 abs max 18 V)
Logic familyBipolar 555 (TTL-compatible at 5 V)
PackagesPDIP-8, SOIC-8, TSSOP-8
Output drive±200 mA
Key thresholdsTrigger ≈ ⅓ VCC, Threshold ≈ ⅔ VCC

Pin configuration

text
             ┌───┴───┐
       GND ─┤ 1   8 ├─ VCC
      TRIG ─┤ 2   7 ├─ DISCH
       OUT ─┤ 3   6 ├─ THRES
     RESET ─┤ 4   5 ├─ CONT
             └───────┘
PinNameTypeDescription
1GNDPowerGround
2TRIGInputTrigger — low (< ⅓ VCC) starts the timing cycle / sets output high
3OUTOutputTotem-pole output, sink/source up to 200 mA
4RESETInputActive-low reset; tie to VCC if unused
5CONTAnalogControl voltage — access to ⅔ VCC divider; often bypassed with 10 nF
6THRESInputThreshold — high (> ⅔ VCC) ends the cycle / resets output low
7DISCHOpen-collectorDischarge path to GND when output is low
8VCCPowerSupply 4.5 V – 16 V

Functional description

Monostable (one-shot)

A falling edge on TRIG drives OUT high and releases DISCH. Capacitor C charges through RA until THRES reaches ⅔ VCC; then OUT goes low and DISCH dumps C.

[ t_{HIGH} \approx 1.1 \cdot R_A \cdot C ]

Astable (oscillator)

THRES and TRIG are tied together. C charges through RA + RB and discharges through RB via DISCH.

[ f \approx \frac{1.44}{(R_A + 2 R_B),C} \quad,\quad Duty \approx \frac{R_A + R_B}{R_A + 2 R_B} ]

Absolute maximum ratings

Warning

Stresses beyond these values cause permanent damage. These are limits, not operating conditions.

ParameterRatingUnit
Supply voltage VCC18V
Input voltage (CONT, RESET, THRES, TRIG)VCCV
Output current±225mA
Operating junction temperature150°C

Electrical characteristics

ParameterSymbolMinTypMaxUnitConditions
Supply voltageVCC4.5—16VNE555 recommended
Supply currentICC—36mAVCC = 5 V, no load
Supply currentICC—1015mAVCC = 15 V, no load
Threshold voltageVTH—⅔ VCC—V
Trigger voltageVTR—⅓ VCC—V
Output sink capabilityIOL——200mA
Reset voltageVRESET0.30.71.0V

Typical application

Astable LED blinker (~1 Hz)

text
VCC (5–15 V)
  │
  ├──── RA (e.g. 10 kΩ) ────┬──── RB (e.g. 100 kΩ) ────┐
  │                         │                          │
  │                      DISCH(7)                   THRES(6)──TRIG(2)
  │                         │                          │
  │                         └────────── C (10 µF) ─────┴── GND
  │
  ├── VCC(8)   OUT(3) ──► LED + series resistor ── GND
  ├── RESET(4) ── VCC
  └── CONT(5) ── 10 nF ── GND
      GND(1) ── GND

Package & footprint

  • PDIP-8 (NE555P): breadboard / through-hole default.
  • SOIC-8 (NE555D): SMT boards; same pin function order.
  • Temperature grades: NE555 0 °C–70 °C, SA555 −40 °C–85 °C, SE555 −55 °C–125 °C.

Common mistakes

  • Leaving RESET floating: Noise resets the timer randomly — always tie pin 4 to VCC if unused.
  • No bypass on CONT: A 10 nF capacitor from pin 5 to GND reduces supply-noise jitter on the thresholds.
  • Expecting rail-to-rail CMOS behavior from bipolar NE555: At 5 V the bipolar output high is well below VCC; use a CMOS 555 (LMC555 / TLC555) for true rail-to-rail and lower quiescent current.
  • Duty cycle stuck above 50% in simple astable: With one discharge resistor (RB only path), duty cannot go below 50%. Add a diode across RB (or use a PWM topology) for asymmetric duty.

Notes

  • Dual version is the NE556 (two timers in one 14-pin package).
  • CMOS drop-ins (TLC555, LMC555) keep the pinout but run from ~2 V with µA-class supply current — preferred for battery projects.

Assets & downloads